PIN diodes are diodes with p-type and n-type regions and an intrinsic region between. In light emitting diodes (LEDs), electrons and holes injected from the p-type and n-type source regions recombine within the intrinsic region, generating light. The type of materials used for the p-type, i-type and n-type regions in the LED determine the wavelength of light emitted from the device.
Simple PIN diodes typically exhibit poor light (radiation) extraction properties which ultimately limits the useful production of light. This is due to the high refractive index of the materials used to make LEDs and the large number of available optical modes within the LED which results in a small escape cone for emitted light from the device. Light generated within the device and having propagation vectors outside this cone do not escape the device and is subjected to total internal reflection, thus being attenuated within the device and not escaping as available light. Typically, known LEDs exhibit approximately less than 5% light extraction efficiency from a planar device, depending primarily upon the refractive index of the material through which the light travels.
One method for increasing efficiency of radiation extraction in such devices is texturing the surface through which radiation exits the device. Such a textured escape surface provides a slightly larger escape cone for radiation from the device. Another method is shaping the top or bottom surface of the LED, such as in a parabolic shape, to improve refraction and reflection of light beams and thus improve light extraction. Alternatively, researchers have also grown epitaxial semiconductors on a textured substrate surface to improve vertical light extraction; however, the penalty in disordered semiconductor crystal structure has severely limited this approach. Coatings can also be used in LEDs to affect emission of certain wavelengths of light, and thus improve the spectral light quality produced by the device.
Different structures of light emitting diodes also present specific design issues for extracting light. For example, edge emitting lasers are one type of light emitting device, where light is propagated in a direction substantially parallel to the device layers and is emitted from the edges of the device. Numerous edge emitting LEDs and lasers are fabricated from a semiconductor wafer and are diced into individual devices, where the cleaved, etched or cut edges become the facet surfaces from which light is emitted. For the case of waveguide LEDs, optical reflection from the facets into the active region is suppressed by Brewster angle configuration, whereas for lasers the facets at either end of the planar cavity must be precisely parallel and of high reflectance. Vertical cavity surface emitting diodes (VCSEDs) and lasers (VCSELs) are another type of light emitting diode where light is propagated substantially perpendicularly to the plane of the device layers and is emitted through the top or bottom surface of the device. The optical cavity can be designed with a resonance to improve emission of a particular wavelength. An advantage of planar VCSEDs and VCSELs is the ability to scale the output power of the device by increasing the planar area of the device. The vertical and lateral optical confinement can also be used to control the optical modes of the device and thus improve the spatial emission profile and spectral quality.